Female mammals carry two copies of the X chromosome as compared to males with an XY complement. This double dosage of X genes must be compensated for and is so by the transcriptional inactivation of one X chromosome. Mistakes in the inactivation process can be observed in human females as skewed inactivation pattern resulting in genetic abnormalties. The long term objective of this research is to broaden the understanding of the process and regulation of mammalian X chromosome inactivation. A key player in the X-inactivation process is the Xist gene. An established genetic assay will be used to study the Xist gene and analyze deletion mutations in order to elucidate the essential domains of the gene and the step in which the product is involved. Systematic overlapping deletions spanning the Xist gene will be created in a yeast vector system. These deletion mutation constructs will be analyzed by a transgenic functional assay for deletions that abolish the inactivation process. Identifying of critical domains within this gene will help to elucidate how the gene functions as well as provide insight to the inactivation process.